Patellar Dislocation – Causes, Symptoms, Treatment

Patellar Dislocation is frequent musculoskeletal injuries seen in the emergency department and represent 3% of all knee injuries. Also, patellar dislocations represent a leading cause of knee hemarthrosis. Patellar dislocation is a distinct entity from a patellar subluxation but occurs in a spectrum of disorders termed patellar instability. Patellar dislocations occur more frequently in the second and third decades of life.

The patella, commonly called the kneecap, is one of the main bones that make up the knee joint and is to the front of the joint. It is a triangular sesamoid bone (i.e. a bone embedded in a tendon) that sits in the patella-femoral groove which is a hollow or notch at the end of the thigh bone (femur) – the end where it meets the shin bone (tibia). It is held in position by a number of tendons and ligaments which let it move up and down the groove when the leg is bent or flexed at the knee joint.

In some cases, the patella slips out of the patellofemoral groove partially (Patellar Subluxation) or totally (Patellar Dislocation) – causing severe pain and affecting the movement of the knee joint. The dislocation is always to the outside of the joint. Athletes are among the most affected group given the nature of their exertions.

The dislocation of the kneecap is different from the dislocation of the knee. Dislocation of knee is when the femur and tibia lose contact. The dislocation of the kneecap is when the kneecap is dislocated from the patellofemoral groove on the femur.

Patellar Dislocation

Anatomy and Physiology of Patellar Dislocation

The patella is a sesamoid bone with proximal attachments to the quadriceps tendon and distal attachments to the patellar ligament. Soft tissues stabilize the patella in extension and the patellofemoral joint guides the patella in knee flexion. MPFL (medial patellofemoral ligament) restrains the patella medially and joins the patella to the medial femoral condyle. MPFL has the primary positioning restraint in the first 20 degrees of flexion. The dynamic stability of the patella is provided by the vastus medialis (VM) muscle.

Anatomical risk factors Patellofemoral instability include increased Q angle (including an increased distance between the trochlear groove and tibial tubercle as measured on advanced imaging modalities), patella alta, patella tilt, trochlear dysplasia, and vastus medialis weakness. The mechanism for dislocation involves valgus stress with a contracted quadriceps muscle.

An unstable patella can dislocate in any direction (proximal, medial, superior, inferior, and vertically), but may also dislocate intraarticularly or on-axis horizontally and vertically (dorsal fin). In the most common type of dislocation, initial x-ray imaging will show the patella translocated laterally, but could also demonstrate contusions on the lateral femoral condyle anteriorly and contusions or avulsion of the medial patella.

Causes of Patellar Dislocation

  • Sudden forceful twist or turn of the knee joint
  • Direct hit or blow on the leg or the knee
  • Congenital predisposition and deformities
  • Faulty alignment of the joint
  • Using inappropriate shoes which do not lend proper support to the knee
  • Excessive stress as in athletes and sportspersons
  • Past injuries of the knee joint or knee cap which did not heal properly
  • Incorrect posture while lifting heavy objects

Symptoms of Patellar Dislocation

  • Swelling, tenderness and inflammation
  • Severe pain immediately on injury – often reported as pain inside the kneecap
  • Visible dislocation of the patella
  • Locking of the knee
  • Impaired and painful mobility of the knee
  • Anterior knee ripping or tearing sensation at injury
  • Knee flexes with dislocation
  • Patella relocates with knee extension
  • Subluxation associated with giving way sensation
  • Dislocation is associated with severe pain


  • Knee held in semi-flexed position
  • Palpable Patella deviated from normal position
    1. Lateral Patellar Dislocation is more common than medial dislocation
  • Dislocation
    1. Concurrent osteochondral Fracture in 28-52% of patients
    2. Associated with Anterior Cruciate Ligament Tear
  • Subluxation
    1. Instability and weakness
    2. Reluctant to bear the weight
  • Predisposing factors
    1. Examine for predisposing factors listed above
    2. J-Sign
    3. Quadriceps angle (Q-Angle) >15 degrees

Diagnosis of Patellar Dislocation

Thorough physical examination of the injured knee and investigation of the patient’s medical history for past injuries, congenital predisposition. Palpation the knee to ascertain the location of damage and the extent of dislocation

Palpatory Exam – Areas of Focus medial aspect of the knee

  • Vastus medialis obliques
  • Superomedial pole patella
  • The medial facet of the patella
  • Origin of the medial collateral ligament (MCL)
  • Midsubstance of the MCL
  • Broad insertion of the MCL
  • Medial joint line
  • Medial meniscus
  • Pes anserine tendons and bursa

Midline knee

  • Quadricep tendon
  • Suprapatellar pouch
  • Superior pole patella
  • Patellar mobility
  • Prepatellar bursa
  • Patellar tendon
  • Tibial tubercle

Lateral aspect of the knee

  • Iliotibial band
  • Lateral facet patella
  • Lateral collateral ligament (LCL)
  • Lateral joint line
  • Lateral meniscus
  • Gerdy’s tubercle

To assess the knee –  a clinician can perform the patellar apprehension test by moving the patella back and forth while the people flex the knee at approximately 30 degrees.[rx]

The people can do the patella tracking assessment by – making a single leg squat and standing, or by lying on his or her back with the knee extended from a flexed position. A patella that slips laterally on early flexion is called the J sign, and indicates an imbalance between the VMO and lateral structures.[rx]


On X-ray with skyline projections, dislocations are readily diagnosed. In borderline cases of subluxation, the following measurements can be helpful:

The lateral patellofemoral angle, formed by[rx]

  • A line connecting the most anterior points of the medial and lateral facets of the trochlea.
  • A tangent to the lateral facet of the patella.

With the knee in 20° flexed, this angle should normally open laterally.[rx]

  • The patellofemoral index – is the ratio between the thickness of the medial joint space and the lateral joint space (L). With the knee 20° flexed, it should measure 1.6 or less.[rx]

MRI scan

  • To be over sure the dislocation and investigate the tendon, cartilage, ligament

Treatment of Patellar Dislocation

The mainstay of treatment for first-time dislocators without evidence of loose bodies or intra-articular damage is conservative, including analgesia, icing, and NSAIDs to reduce pain and swelling, physiotherapy, and activity modification. Bracing in a J brace or a patella stabilizing sleeve may be beneficial short term (2 to 4 weeks) to allow the soft tissues to heal. Subsequent, physiotherapy should be started with an emphasis on quadriceps and vastus medialis oblique strengthening, core strengthening, and proprioception. The patient can be allowed to weight bear as tolerated.

  • The doctor may suggest avoiding any stress on the knee joint/kneecap
  • The knee should be rested – keeping the leg elevated at the level of the chest
  • Painkillers and anti-inflammatory medicines may be prescribed
  • Compression with a soft bandage
  • Ice packs to prevent/reduce swelling
  • Immobilization of the knee joint using a cast or a knee brace for a couple of weeks
  • Physiotherapy may strengthen the hamstring and quadriceps
  • Orthotic devices may be inserted in shoes to lend support to the knee joint/patella
  • Special tape to improve the alignment and stability of the knee joint

Surgical Management

Surgery in extreme cases, recurrent injuries because of improperly healed injuries in the past or because of congenital abnormalities

Surgical management can be a consideration in several situations:

  • A first time dislocation with osteochondral fracture/loose body
  • MRI demonstrating disruption of MPFL
  • Subluxed patella on Merchant radiograph view with a normal contralateral knee
  • Failure to improve with conservative management with anatomical factors which predispose to dislocation
  • Recurrent dislocations

There is evidence that suggests that early stabilization procedures can reduce the rate of subsequent dislocations but in the absence of clear subjective benefits at long term follow up.Surgical management is usually via proximal and distal realignment. There are numerous surgical options available:

Arthroscopy with or without open debridement

  • Arthroscopic or open debridement with removal of any loose bodies may be necessary for displaced osteochondral fractures or loose bodies. Osteochondral fragment repair is favored if sufficient bone is available for fixation.

MPFL repair (re-attachment) or reconstruction (proximal realignment)

  • Proximal realignment constitutes repairing or reconstructing the MPFL. In brief, to repair the ligament, a longitudinal incision is made at the border of the VMO, just anterior to the medial epicondyle. The ligament is usually re-attached to the femur using bone anchors. If the patient has had recurrent dislocations, then reconstruction of the MPFL is necessary using gracilis or semitendinosus autografts, or allografts.
  • Isolated repair/reconstruction of the MPFL is not a recommendation in patients with bony abnormalities including TT-TG distance greater than 20mm, convex trochlear dysplasia, severe patella alta, advanced cartilage degeneration orsevere femoral anteversion.

Lateral release (distal realignment)

  • A lateral release cuts the retinaculum on the lateral aspect of the knee joint. The aim is to improve the alignment of the patella by reducing the lateral pull. It is not performed in isolation, but usually in combination with other more powerful realignment procedures.

Osteotomy (distal realignment)

  • An osteotomy is necessary when there is abnormal anatomy contributing to poor patella tracking and a high TT-TG distance. The most common procedure of this type is known as the Fulkerson osteotomy.
  • This osteotomy involves cutting the anterior tibia at a certain angle to allow for anterior and medial displacement of the osteotomized fragment, moving the patellar tendon insertion on the tibia (along with the patella) more medially and anteriorly.
  • This will reduce the Q angle and will decrease the risk of recurrence. If the patient has patella alta, an osteotomy allows the surgeon to effectively lower the patella by distal displacement of the osteotomized fragment.
  • In the presence of a rotational deformity, a derotational osteotomy of the femur may be considered. Those procedures are not appropriate in patients with open growth plates.


  • Trochleoplasty is indicated in recurrent dislocators with a convex or flat trochlea. The trochlear groove is deepened to create a deeper groove for the patella to glide through; this may take place in conjunction with an MPFL reconstruction.
  • Studies have suggest that trochleoplasty is not advisable in patients with open growth plates or severely degenerative joint. This procedure is uncommon but may be considered in severe cases.

Postoperative and Rehabilitation Care

The post-operative rehabilitation protocol will vary based on the surgeon, but the following is a general guideline.

Weeks 0 through 2

  • Goal – protect surgical repair of tendon
  • Weight-bearing as tolerated with crutches and knee brace locked in full extension
  • Operating surgeon determines ROM allowances based on the quality of repair

Weeks 2 through 6

  • Goal – continue protecting surgical repair of tendon, normalize gait with crutches and knee brace
  • Continue weight bearing as tolerated with crutches and knee brace locked in full extension
  • Begin passive ROM from 0 to 90 degrees of knee flexion, no active quadriceps extension
  • Operating surgeon determines ROM allowances based on the quality of repair

Weeks 6 through 12

  • Goal – normalize gait on a flat surface, wean crutches, the knee brace may be opened to allow flexion, begin active quadriceps contraction
  • Gradual progression of weight bearing with knee flexion, avoid weight bearing in knee flexed past 70 degrees
  • Active ROM of knee
  • Progressive light squat, leg press, core strengthening, and other physical therapy exercises and modalities

Weeks 12 through 16

  • Goal – normalize gait on all surfaces without a brace, full ROM, single leg stance with good control, and squat to 70 degrees of flexion with good control
  • Begin non-impact balance and proprioceptive drills
  • Continue with physical therapy exercises, quad, and core strengthening

Weeks 16 and Longer

  • Goal – good quad control, no pain with sport or work specific movement including impact activity

Return to Sport Criteria

  • Dynamic neuromuscular control with multiplane activities, without pain or swelling

Complications of Patellar Dislocation

  • Re-rupture
  • Residual extensor mechanism weakness
  • Residual extensor lag or inability to fully extend the knee
  • Knee stiffness
  • Quadriceps atrophy
  • Infection


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